首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   708篇
  免费   60篇
  国内免费   48篇
测绘学   4篇
大气科学   82篇
地球物理   137篇
地质学   95篇
海洋学   189篇
天文学   22篇
综合类   26篇
自然地理   261篇
  2023年   2篇
  2022年   11篇
  2021年   17篇
  2020年   23篇
  2019年   19篇
  2018年   20篇
  2017年   32篇
  2016年   26篇
  2015年   22篇
  2014年   41篇
  2013年   72篇
  2012年   32篇
  2011年   43篇
  2010年   25篇
  2009年   41篇
  2008年   48篇
  2007年   47篇
  2006年   42篇
  2005年   33篇
  2004年   30篇
  2003年   22篇
  2002年   29篇
  2001年   21篇
  2000年   21篇
  1999年   9篇
  1998年   12篇
  1997年   7篇
  1996年   16篇
  1995年   7篇
  1994年   9篇
  1993年   4篇
  1992年   5篇
  1991年   3篇
  1990年   2篇
  1989年   1篇
  1988年   2篇
  1987年   2篇
  1986年   1篇
  1985年   2篇
  1984年   2篇
  1983年   7篇
  1982年   2篇
  1981年   3篇
  1979年   1篇
排序方式: 共有816条查询结果,搜索用时 46 毫秒
111.
极端事件对人类系统的影响   总被引:1,自引:0,他引:1  
在IPCC特别报告《管理极端事件和灾害风险,推进气候变化适应》中,极端天气气候事件对人类系统的影响是最重要的影响评估内容之一,其评估结果为:极端影响可能缘于极端天气气候事件,但也可能并非极端事件的后果。暴露度和脆弱性是灾害风险的重要决定因素;极端和非极端天气气候事件的严重程度和影响在很大程度上取决于对这些事件的脆弱性和暴露度水平;人居模式、城市化和社会经济状况的变化已经影响观测到的脆弱性和暴露度的变化趋势;无论在发达国家还是发展中国家,沿海人居环境均暴露于极端事件,并受其影响,如小岛屿国家和亚洲大三角洲地区;脆弱人口还包括难民、国内流离失所的人和那些生活在边远地区的人;极端事件将极大地影响与气候联系密切的部门,如水、农业、食物安全、健康和旅游业。  相似文献   
112.
Levee effects upon flood levels: an empirical assessment   总被引:1,自引:0,他引:1  
This study used stream gauge records to assess the impact of levees on flood levels, providing an empirical test of theoretical and model predictions of the effects on local flood response. Focusing upon a study area in Illinois and Iowa for which levee records were available, we identified 203 gauges with ≥ 50 years hydrological record, including 15 gauges where a levee was constructed during the period of record. At these sites, step‐change analysis utilizing regression residuals tested levee‐related stage changes and levels of significance and quantified the magnitudes of stage changes. Despite large differences in stream sizes, levee alignments, and degree of floodplain constriction, the post‐levee rating‐curve adjustments showed consistent signatures. For all the study sites, stages for below bankfull (non‐flood) conditions were unaffected by levee construction. For above bankfull (flood) conditions, stages at sites downstream of their associated levees also were statistically indistinguishable before versus after levee construction. However, at all sites upstream of levees or within leveed reaches, stages increased for above bankfull conditions. These increases were abrupt, statistically significant, and generally large in magnitude – ranging up to 2.3 m (Wabash River at Mt. Carmel, IL). Stage increases began when discharge increased above bankfull flow and generally increased in magnitude with discharge until the associated levee(s) were overtopped. Detailed site assessments and supplementary data available from some sites helped document the dominant mechanisms by which levees can increase flood levels. Levee construction reduces the area of the floodplain open to storage of flood waters and reduces the width of the floodplain open to conveyance of flood flow. Floodplain conveyance often is underestimated or ignored, but Acoustic Doppler Current Profiler (ADCP) measurements analysed here confirm previous studies that up to 70% or more of the total discharge during large floods (~3% chance flood) can move over the floodplain. Upstream of levees and levee‐related floodplain constriction, backwater effects reduce flow velocities relative to pre‐levee conditions and, thus, increase stages for a given discharge. The empirical results here confirm a variety of theoretical predictions of levee effects but suggest that many one‐dimensional model‐based predictions of levee‐related stage changes may underestimate actual levee impacts. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   
113.
Addressing social and economic considerations is crucial to the success of Marine Protected Area (MPA) planning and management. Ineffective social assessment can alienate local communities and undermine the success of existing and future MPAs. It is rare to critique the success of methods used currently to incorporate social and economic considerations into MPA planning. Three Australian MPA planning processes covering three states and incorporating federal and state jurisdictions are reviewed in order to determine how potential social impacts were assessed and considered. These case studies indicate that Social Impact Assessment (SIA) is under-developed in Australian MPA planning. Assessments rely heavily on public participation and economic modelling as surrogates for dedicated SIA and are followed commonly by attitudinal surveys to gauge public opinion on the MPA after its establishment. The emergence of issues around public perception of the value of MPAs indicates the failure of some of these proposals to adequately consider social factors in planning and management. This perception may have potential implications for the long term success of individual MPAs. It may also compromise Australia's ability to meet international commitments for MPA targets to gazette at least 10% of all its marine habitats as MPAs. Indeed, this is demonstrated in two of the three case studies where social and economic arguments against MPAs have been used to delay or block the future expansion of the MPA network.  相似文献   
114.
季菲菲  陈雯  袁丰  孙伟 《地理研究》2013,32(10):1899-1911
科技金融是伴随着中国科技体制改革和金融发展不断深化、服务于自主创新和创新型国家建设战略而逐渐产生的新生事物。以中国科技金融创新的主要发源地之一的无锡国家级高新区为例,通过实地调查,对科技金融体系的产生、发展及其空间效应进行实证研究。研究发现,无锡新区的科技金融服务已基本形成一个完整的体系;政策、市场和社会因素是科技金融发展的主要推动力;科技金融体系的建立推动了产业转型,专门的科技金融空间出现,同时在科技金融支持下新的产业空间出现,并引发人口空间转移和社会空间转型等。本文可丰富科技金融和高新区转型的理论,并为其他地区的科技金融实践提供借鉴。  相似文献   
115.
研究季节性放牧对植被耗水量、水分利用效率的影响,是探索如何提高高寒草甸水源涵养能力的重要内容之一。以青藏高原三江源高寒草甸季节性放牧样地与自然放牧样地为研究对象,分析了季节性放牧和自然放牧条件下高寒草甸植被耗水量、水分盈亏量、水分利用效率(WUE)的动态变化及其与环境因素的关系。结果表明:在植被生长季(5-9月),季节性放牧样地和自然放牧样地植被耗水量在5月开始增加, 7月达最高,分别为160.94 mm和145.96 mm,季节性放牧样地植被总耗水量(395.52 mm)比自然放牧样地(348.14 mm)高13.61%。生长季平均来看,季节性放牧样地和自然放牧样地5-9月水分正盈余,分别为13.58 mm和70.96 mm,但在植物生长旺季(8月)略有亏缺。季节性放牧样地和自然放牧样地植被耗水量均与降水量呈弱的正相关关系。季节性放牧样地植被地上净初级生产量(ANPP)、地下净初级生产量(BNPP)和总的净初级生产量(NPP)比自然放牧样地分别高32.54 g·m-2、5.96 g·m-2、38.50 g·m-2,季节性放牧样地ANPP的水分利用效率(WUE)比自然放牧样地高53.85%,而BNPP、NPP的WUE比自然放牧样地分别低13.06%和9.97%。这表明,季节性放牧可提高植被生产量和耗水量,但对高寒草甸WUE的影响因放牧方式不同导致地上、地下生物量分配格局不同而有所差异。  相似文献   
116.
1961-2005年水利水保措施对潮河流域年径流量的影响(英文)   总被引:1,自引:0,他引:1  
Taking the Chaohe River Basin above the Miyun Reservoir in North China as a study area,the characteristics and variation trends of annual runoff and annual precipitation during 1961-2005 were analyzed applying Mann-Kendall test method on the basis of the hydrologic data of the major hydrological station(Xiahui Station) located at the outlet of the drainage basin and the meteorological data of 17 rainfall stations.Human activities including water conservancy projects construction and water diversion as well as implementation of soil and water conservation from 1961 to 2005 were carefully studied using time series contrasting method.The referenced period(1961-1980) that influenced slightly by human activities and the compared period(1981-2005) that influenced significantly by water conservancy and soil conservation measures were identified according to the runoff variation process analysis and abrupt change points detection during 1961-2005 applying double accumulative curve method,mean shift t-test method and Mann-Kendall mutation test technique.Based on the establishment of a rainfall-runoff empirical statistical model,impacts and the runoff-reducing effects of water conservancy and soil conservation measures on runoff reduction were evaluated quantitatively.The major results could be summarized as follows:(1) The annual precipitation in the drainage basin tends to decrease while the runoff has declined markedly since the 1960s,the average annual runoff from 1991 to 2000 was only 90.9% in proportion to that from 1961 to 1970.(2) The annual runoff variations in the drainage basin are significantly related to human activities.(3) During 1981-1990,1991-2000,2001-2005 and 1981-2005,the average annual runoff reduction amounts were 1.15×108,0.28×108,1.10×108 and 0.79×108 m3 respectively and the average annual runoff-reducing effects were 31.99%,7.13%,40.71% and 23.79% accordingly.Runoff-reducing effects by water conservancy and soil conservation measures are more prominent in the low water period.  相似文献   
117.
Taking the Chaohe River Basin above the Miyun Reservoir in North China as a study area, the characteristics and variation trends of annual runoff and annual precipitation during 1961–2005 were analyzed applying Mann-Kendall test method on the basis of the hydrologic data of the major hydrological station (Xiahui Station) located at the outlet of the drainage basin and the meteorological data of 17 rainfall stations. Human activities including water conservancy projects construction and water diversion as well as implementation of soil and water conservation from 1961 to 2005 were carefully studied using time series contrasting method. The referenced period (1961–1980) that influenced slightly by human activities and the compared period (1981–2005) that influenced significantly by water conservancy and soil conservation measures were identified according to the runoff variation process analysis and abrupt change points detection during 1961–2005 applying double accumulative curve method, mean shift t-test method and Mann-Kendall mutation test technique. Based on the establishment of a rainfall-runoff empirical statistical model, impacts and the runoff-reducing effects of water conservancy and soil conservation measures on runoff reduction were evaluated quantitatively. The major results could be summarized as follows: (1) The annual precipitation in the drainage basin tends to decrease while the runoff has declined markedly since the 1960s, the average annual runoff from 1991 to 2000 was only 90.9% in proportion to that from 1961 to 1970. (2) The annual runoff variations in the drainage basin are significantly related to human activities. (3) During 1981–1990, 1991–2000, 2001–2005 and 1981–2005, the average annual runoff reduction amounts were 1.15×108, 0.28×108, 1.10×108 and 0.79×108 m3 respectively and the average annual runoff-reducing effects were 31.99%, 7.13%, 40.71% and 23.79% accordingly. Runoff-reducing effects by water conservancy and soil conservation measures are more prominent in the low water period.  相似文献   
118.
医学地理研究由来已久,但地理学界血吸虫病医学地理研究相对薄弱,而且多是非地理专业人员在从事相应工作。本文首先从血吸虫病流行传播的机理出发,分析了血吸虫病医学地理的主要研究内容,认为钉螺滋生地与血吸虫病疫情的监测和预测是血吸虫病医学地理研究的主要内容;其次系统总结了气象气候因子、土壤植被因子、地貌高程因子、水文水力因子、人文经济因子对血吸虫生命史及其宿主的影响,并视其为血吸虫病医学地理研究的基础;然后回顾了近年国际、国内运用遥感、GIS技术和统计学方法(包括地统计学方法)进行钉螺滋生地与血吸虫病疫情确定的主要研究手段,归纳了7类钉螺滋生地预测方法和5类血吸虫病疫情预测方法;最后剖析了目前血吸虫病医学地理研究面临的主要挑战和未来血吸虫病医学地理研究努力方向。  相似文献   
119.
准噶尔盆地南缘古牧地地区具有丰富的油气勘探潜力,但由于中深层地震成像存在盲区,长期以来对古牧地构造特征认识不清,制约了油气勘探进程.笔者采用高精度重磁和电法CEMP综合物探技术对古牧地地区构造特征进行了研究,取得了有价值的新认识.研究成果表明,古牧地构造为博格达山前褶皱冲断带的前锋构造,受阶梯状断裂控制.古牧地构造纵向上具多层构造特征,其中浅层为紧闭型逆冲倒转背斜,中层为受阶状逆冲断层控制的屋脊状断块,深层为潜山披覆型冲断背斜.重力异常显示古牧地背斜西围斜可能是一完整的背斜构造,古牧地中深层构造和古牧地背斜西围斜评价为有利的油气勘探目标.  相似文献   
120.
Although geographers are highly visible in research into contemporary climate changes accompanying greenhouse gas increases (“global warming”), a relatively little explored component by them is the impact of aviation. This contrasts with the atmospheric science community of cloud physicists, chemists, and numerical modelers, who use geographic tools of trade. Geographers are well positioned to investigate aviation-climate impacts, especially related to contrail clouds, given geographic traditions of environment, spatial variation, regionalization, generalization, and the integrative approach to studying physical and human systems (here, climate, transportation, and energy). We highlight the role played by geographers in contrail-climate studies and advocate for a broader involvement in aviation policy formulation.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号